(a) Field of the Invention:
The present invention relates to a fuel control system for air-fuel mixture supply devices, said system being of the type that there are formed separately and independently relative to each other a negative pressure which serves as the signal of the flow rate of the intake mixture travelling through the intake mixture passageway for supplying a mixture to an engine, and a negative pressure source for the intake of the fuel into the intake mixture passageway.
(b) Description of the Prior Art:
Supposing here, for the convenience of explanation, a carburetor as a simple-structured mixture supplying device as shown generally in FIG. 1, the air-fuel ratio of the mixture formed by the carburetor will theoretically be kept constant despite the fluctuations of the air flow rate. More particularly, in FIG. 1, as air flows in the direction indicated by an arrow 3 through a main intake mixture passageway 1 at a flow rate which is proportional to the degree of opening of a throttle valve 2, a negative pressure proportional to the square of the air flow rate develops in a venturi section 4, while a pressure difference which is proportional to the square of the air flow rate will develop on both sides of a fuel metering jet 5, so that a fuel having a flow rate proportional to the air flow rate is to be supplied into the main intake mixture passageway 1.
In the actual practice, however, the air-fuel ratio is not one which is so simply determined as stated above. That is, a fuel passageway 7 extending from the fuel metering jet 5 to a fuel nozzle 6 which opens into the venturi section 4 has a substantial length relative to the cross-sectional area of the fuel passageway 7, and accordingly, there will develop a resistance of flow of fuel in magnitude proportional (or relative) to the fuel flow rate, or in some cases the flow will become a laminar flow and on some other cases will present a turbulent flow due to the fluctuation of the flow rate owing to the changes in Reynolds number caused by the fluctuations of the flow speed. Moreover, the fuel passageway 7 is not necessarily formed in a straight line, and accordingly the state of flow will not become constant. Also, in case a bleed air is introduced, the flow resistance will exert variations in a complex fashion.
As such, for each different design of carburetor or for each different engine structure which uses a carburetor, there has been the need, in the past, to take time and labor in selecting the size of respective functioning parts which jointly constitute a fuel control system.
In the past, there have been proposed various types of electronically controlled carburetors in an effort to keep the air-fuel ratio of the mixture at a certain theoretical value for all operating conditions. As one such example, there has been known an electronically controlled carburetor which is arranged to effect a duty-control of bleed air via an electromagnetic valve by providing O.sub.2 sensor for detecting the oxygen content in the exhaust gas to input, into a computer, the signal which is outputted from this O.sub.2 sensor as well as various operating parameters of the engine, so as to be able to determine the fuel flow rate to insure that the air-fuel ratio of the mixture will assume a certain theoretical value by virtue of the output signal coming from the computer as the result of computation performed based on the above-mentioned various inputted data. In such an electronically controlled carburetor as mentioned above which is designed to control the fuel flow rate by feeding back the signals delivered from the O.sub.2 sensor, there is the need to input, into respective controlling circuits, a large number of parameters of operation of the engine in order to establish a coincidence between the air-fuel ratio and the theoretical value. Therefore, in addition to the large number of setting-work steps required in the conventional carburetors, there have to be added further work steps for the above said feedback controlling in addition to the conventional setting-work steps. This will bring about an undesirable increase in the number of work steps, leading to an increase in the manufacturing cost. Also, there is known an electronically controlled constant vacuum type carburetor arranged so that the intake mixture flow rate in the main mixture passageway is inputted into the controlling circuit as an electric signal based on the amount of lift of the vacuum piston, to operate an electrostriction vibrator by the output from said controlling circuit, whereby to effect duty-control of the valve-opening time of the main fuel passageway (Japanese Patent Preliminary Publication No. Sho 56-47649). The most obvious problem noted of this type of electronically controlled carburetor lies in that, because of the presence of a frictional resistance on the sliding surface of the vacuum piston, there exists, at a position in the direction of sliding of the vacuum piston, a hysteresis for the negative pressure variations in the variable venturi section and that, therefore, the detection signal of the position of the vacuum piston fails to indicate a correct air flow rate. Also known is an electronically controlled constant vacuum type carburetor arrange so that there are inputted to the controlling circuitry an electric signal based on the amount of lift of the vacuum piston and representing the air flow rate in the main mixture passageway, and also an electric signal based on the difference in pressure between the up-stream side and the down-stream side of main fuel metering jet serving as a feedback signal of the fuel flow rate, so that a stepping motor is driven by the pulse output delivered from said controlling circuitry, whereby the amount of the bleed air is controlled (Japanese Patent Preliminary Publication No. Sho 57-124062). In this latter instance also, the amount of lift of the vacuum piston fails to correctly indicate the air flow rate, just as in the preceding instance mentioned above. Also, in the constant vacuum type carburetor, the negative pressure fluctuations which can take place therein are few, and the pressure difference between the up-stream side and the down-stream side of the main fuel metering jet is controlled mainly by the bleed air flow rate, so that there arises the drawback that the controlling of fuel cannot be performed satisfactorily. Furthermore, there is known a constant vacuum type carburetor arranged so that electric signals based on the temperature of the cooling water, the rotation speed of the engine and the negative pressure of the mixture passageway, respectively, of the engine are inputted to the controlling circuitry, and that by the pulse output delivered from said controlling circuitry, the bleed air is duty-controlled via an electromagnetic valve, whereby to control the fuel flow rate (Japanese Patent Preliminary Publication No. Sho 60-43160). In this instant case, the vacuum piston does not possess a lift amount sensor. However, in spite of the fact that the above-mentioned variance of the amount of lift brings about variance of the negative pressure generated in the variable venturi section which, in turn, is formed by the vacuum piston, there is contained in the inputs delivered to the controlling circuitry no element which compensates for the variance of the negative pressure. Thus, a correct control cannot be expected in this prior art either. Further, in the system disclosed in U.S. Pat. No. 4,201,166, like the present invention, the fuel flow ratio is controlled by utilizing the pressure difference between the fuel pressure in the down-stream side of the main fuel metering jet and the negative pressure in the venturi section, but generally, it cannot be expected to obtain sensors having high precision for detecting the aforesaid pressure difference and besides, the intake of the negative pressure is located in the venturi section in which the main nozzle is opened, and the venturi section is exposed to fuel. Accordingly, this system has the drawback that the fuel enters into the negative pressure passageway and the detection of the negative pressure becomes inaccurate.